The invention relates to a circuit arrangement for delivering power to coils, notably to gradient coils of an MR apparatus, which circuit arrangement includes at least a first coil whose magnetic field interacts with that of at least a second coil, both coils being traversed by a current I, and also includes at least a first and a second voltage source and at least a first and a second quadrupole switching network which are provided with inputs for control and regulating signals via which the current I through the quadrupole switching network can be controlled, the two terminals at the input of the first quadrupole switching network being connected to the first voltage source in a potential embracing fashion and the second voltage source being connected to at least one terminal at the input of the second quadrupole switching network.
Magnetic resonance imaging methods play a crucial role in contemporary medical diagnostics. Such an imaging method is not hazardous to the patient and delivers spatial information concerning the examination volume and also offers sufficiently exact information as regards the types of tissue involved.
To this end the magnetic resonance imaging device utilizes the spins of the nuclei of hydrogen atoms which occur as protons in the tissue. Using external stationary and non-stationary magnetic fields produced by the MR apparatus, part of the protons is excited in such a manner that the emitted electromagnetic radiation provides information concerning the location of the emission on the basis of its frequency and phase. In order to enable individual types of tissues to be reliably distinguished, various characteristics of the tissue are used. Most important in this respect is the decay of the excitation while emitting energy, or in other words the longitudinal relaxation, and the dephasing of the spins due to interactions between the individual protons, that is, the transverse relaxation.
In the context of a typical magnetic resonance imaging method the magnetic moment of the protons is oriented in one spatial direction by means of a strong, stationary magnetic field of approximately 1.5 Tesla. Using brief electromagnetic RF pulses, the individual protons are excited to precession and are subsequently oriented again in conformity with the external, strong magnetic field. Notably the times for excitation and relaxation as well as the frequencies of the precessions are dependent on the tissue and in the context of the measurement they provide, in conjunction with spatial encoding of the excitation, information concerning the situation in space of different tissues. The spatial encoding utilizes location-dependent frequencies and phases of the precessional excitation and offers, via a Fourier transformation of the measured MR signal, information concerning the location of the relevant emission.
For a good image quality it is useful to utilize several characteristics of the relevant types of tissue in order to distinguish these types. A restrictive factor in achieving a suitable image quality within an acceptable examination time is the speed at which the magnetic fields in the MR apparatus can be varied. Therefore, it is an on-going aim to develop coil systems which enable high magnetic field strength transients in conjunction with amplifiers and voltage sources.
For the delivery of power to gradient coils it is already known to connect two amplifiers in series with the coils to be powered; one of these two amplifiers is customarily very powerful and generates the necessary voltage of the desired order of magnitude, whereas the other amplifier compensates any inaccuracies occurring. The output stages of the amplifiers are usually constructed as a full bridge by means of power transistors. These transistors are usually IGB transistors (Insulated Gate Bipolar transistors) or MOSFET transistors (Metal Oxide Semiconductor Field Effect Transistors) which are switched at frequencies of between 20 kHz and 100 kHz in order to realize the desired currents and current transients. The power amplifier generates the necessary voltages above all in the phases of high current transients, whereas the full bridge associated with the amplifier is connected in the forward direction in the phases of constant current.
The known circuit arrangements for delivering power to MR gradient coils necessitate the presence of separate voltage sources for each individual power amplifier. Because a conventional MR apparatus is always provided with three gradient coils in conformity with the three spatial co-ordinates, the manufacture of such an arrangement is very expensive. The conventional circuit arrangements have a further advantage in that the construction is usually asymmetrical so that a significant amount of insulation is required.
Considering the drawbacks of the present state of the art, it is an object of the invention to provide a circuit arrangement for delivering power to coils which enables the generation of as fast as possible magnetic field sequences in the MR apparatus and can be manufactured more economically at the same time. Furthermore, the circuit arrangement should have a modular construction which enables extensions and larger assemblies while at the same time reducing the number of components.
This object is achieved in accordance with the invention by means of a circuit arrangement for delivering power to coils of the kind set forth in which the two terminals at the output of the first quadrupole switching network are connected, via the first coil and the second coil, respectively, to the two terminals at the output of the second quadrupole switching network and in which the current I flows from the first quadrupole switching network, via the first coil, to the second quadrupole switching network and from the second quadrupole switching network, via the second coil, to the first quadrupole switching network.
A special advantage resides in the fact that a common voltage source with a common capacity can be provided so as to deliver power to a plurality of coil systems. The voltage sources customarily provided for the X direction, the Y direction and the Z direction can thus be replaced by a single voltage source which is available for each of the spatial directions as required. The first amplifier, consisting of the first voltage source and the first quadrupole switching network is usually not a very powerful type and is subject to small voltage transients only during switching events in the usually powerful second amplifier which consists of a second voltage source and a second quadrupole switching network, so that this first amplifier can be constructed more economically. A further advantage of the circuit arrangement resides in the inherent symmetry in the powering of the coils. At the same time the risk of damage in the first quadrupole switching network or the first voltage source is reduced in the case of faults in the second quadrupole switching network. The voltage sources used, moreover, can be advantageously arranged symmetrically with respect to ground, so that the insulation required for all components of the MR apparatus is reduced. The modular construction of the circuit arrangement, moreover, offers an advantageous possibility for refitting at a later stage, notably the refitting of the second quadrupole switching network with the second voltage source which together, as a power amplifier, advantageously extend the possibilities for use of the MR apparatus.
In an advantageous further embodiment of the invention, under the control of control signals at least one quadrupole switching network reverses the pole of the voltage at the output of the quadrupole switching network relative to the voltage at the input and/or decouples and/or chokes and/or amplifies this voltage and/or the terminals at the input and/or the terminals at the output can be short-circuited among themselves and/or to one another. In conjunction with the circuit arrangement in accordance with the invention, such switching features offer new degrees of freedom in delivering power to the coils which make optimum use of the power of the voltage sources and at the same time enable exact control of the delivery of power.
For a single-channel version of the circuit arrangement it is advantageous that the second voltage source is connected to the two terminals at the input of the second quadrupole switching network in a potential embracing fashion. The first voltage source is advantageously conceived for fine adjustment of the delivery of power to the coil and the second voltage source for generating the necessary order of magnitude of the coil current. Both voltage sources are customarily connected in parallel with a capacitance in order to avoid voltage surges. The symmetry of the circuit arrangement in respect of the two voltage sources and the resultant minimum insulation requirements are particularly advantageous.
For a multichannel construction of the circuit arrangement it is advantageous that voltage sources are connected in series each time with the first voltage source and that the total voltage is applied to the inputs of k parallel second quadrupole switching networks, that kxe2x88x921 further first quadrupole switching networks are connected parallel to the first quadrupole switching network, that a first quadrupole switching network is associated each time with a second quadrupole switching network, that the terminals of the outputs of the first quadrupole switching networks are connected each time to an output of the associated second quadrupole switching network, each time via a coil, and that the two magnetic fields of the coils which connect the associated quadrupole switching networks to one another interact with one another. The advantages in accordance with the invention, notably in respect of the possibilities of extension and the modular configuration of the circuit arrangement, are particularly advantageous in the case of an extension to a multichannel configuration. Further channels can be added without modification of the initial configuration and the voltage sources present therein are advantageously used in common.
In order to ensure that voltage surges at the voltage sources are practically precluded, it is advantageous that j voltage sources can be connected, via a respective switch, in parallel with the first voltage source, that the parallel connected voltage is applied to the inputs of i+1 parallel second quadrupole switching networks, that i further first quadrupole switching networks are arranged parallel to the first quadrupole switching network, that a first quadrupole switching network is associated each time with a second quadrupole switching network, that the terminals of the outputs of the first quadrupole switching networks are connected each time to a terminal at the output of the associated second quadrupole switching network, each time via a coil, and that the two magnetic fields of the coils which interconnect the associated quadrupole switching networks interact with one another. The circuit arrangement can thus satisfy very severe requirements as regards voltage stability, even in the case of large currents.
In order to minimize the required amount of insulation, it makes sense when the number k of the additional parallel connected second quadrupole switching networks, the number i of the additional first quadrupole switching networks and the number j of the additional voltage sources which can be connected parallel to the first voltage source are even and when the arrangement of the additional first voltage sources, additional first quadrupole switching networks and the additional second quadrupole switching networks is configured so as to be symmetrical relative to the first voltage source or the first quadrupole switching network and the associated second quadrupole switching network. Such a symmetrical configuration on the one hand minimizes the amount of insulation required for all components of the MR apparatus and on the other hand ensures powerful feeding of the coils.
In order to enable the voltage requirements for the delivery of power to the coils to be met at all times, it is advantageous that the voltage sources are connected, via switches, in series with one another and with the first voltage source, and that when the switches are closed, the total voltage is applied to the inputs of the k parallel second quadrupole switching networks and the switches for parallel connection are opened by control when the switches for series connection are closed and the switches for series connection are opened by control when the switches for parallel connection are closed. This arrangement fully utilizes the capacity of the voltage sources and offers additional possibilities and degrees of freedom for control of the delivery of power to the coils.
In order to minimize also energy losses, it is advantageous that the two terminals at the output of at least one second quadrupole switching network are connected to one another via switches whereby the two coils can be connected in series in direct succession notably during the phases of constant current. The comparatively high dissipation of loss heat from the corresponding amplifier can be avoided at least partly during the phases of constant current.
The switches used for the parallel or series connection of the voltage sources and the switches used for the series connection of the coils are advantageously transistors, preferably IGB transistors or MOSFET transistors. It also makes sense to use IGB transistors or MOSFET transistors as the switches of the quadrupole switching networks.
In conjunction with the novel circuit arrangement it is advantageous that one of the four quadrupole switching networks at the input is connected each time to at least two transistors at both terminals, and that the transistors at the output side are connected pair-wise together to the two terminals at the output in a pole embracing fashion. The transistors, advantageously formed by IGB transistors or MOSFET transistors, allow for switching frequencies of 100 kHz and higher, so that a switching network of this kind is particularly well suitable for the required control of the delivery of power to the coils.